Conventionally, lift cylinder supporting elements for supporting one end portion of a lift cylinder for vertically raising and lowering a working machine (a blade in a bulldozer) are provided at the front portions of a pair of left and right main frames composing a main part of a vehicle body of construction equipment (more particularly, a bulldozer). The pair of left and right main frames are connected by a cross member vertically swingably supporting an equalizer bar for coupling a pair of left and right track frames with a traveling crawler wounded thereon. In an inside frame type in which the left and right end portions of a working machine frame (a U-shaped, or a C-shaped frame) mounted with a working machine are respectively located between the left and right main frames and track frames, the working machine frame supporting elements for vertically swingably supporting the aforesaid working frame is provided at the front portions of the main frames. In an outside frame type in which the left and right end portions of the working machine frame are located outside the left and right track frames, the working machine frame supporting elements are provided on the outside of the truck frames.
A steering case, which is internally provided with a steering device and has mounting faces to power plant (a final reduction gear cover and the like) and mounting faces to supporting members for a traveling device (a track frame and the like), is welded to the rear portions of the pair of left and right main frames to be a single-piece structure. The steering case consists of a structure of welded sheet metal, or a work piece of cast steel.
External forces exerted on the blade during operation act on the main frames via the working machine frame and the lift cylinder. Accordingly, the main frames need to have enough strength and rigidity to withstand the external forces, and thus various proposals have been conventionally made regarding the main frame structure for this purpose. As for such a main frame structure, for example, Japanese Utility Model Laid-open No. 2-88877 and Japanese Patent Publication No. 62-24583 have been known.
FIG. 19 is a perspective view showing a main frame structure disclosed in Japanese Utility Model Laid-open No. 2-88877, and FIG. 20 is a side view showing a situation in which a working machine frame is attached to the main frame. As shown in FIG. 19, a main frame 71 is divided into a pair of main frame front portions 72 and a pair of main frame rear portions 73. A cross bar 74 (hereinafter referred to as a cross member 74) is connected with members 77a, 77b, and 77c which connect a front face plate 75 and a rear face plate 76. The main frame front portions 72 are attached to the front face plate 75 of the cross member 74 in a state in which the main frame front portions 72 are inwardly offset with the width thereof smaller than that of the main frame rear portions 73. On the other hand, the main frame rear portions 73 are attached to the rear face plate 76 of the cross member 74. A working machine bracket 13 is attached to the front face plate 75 of the cross member 74 outside the main frame front portion 72 with bolts. The working machine bracket 13 swingably supports a working machine frame 32 to which a blade 31 is attached, and corresponds to a working machine frame supporting member 13 or 105 (see FIG. 1 or FIG. 15) of the present invention described later. A cylinder bracket 79 is attached on the top face of the main frame rear portion 73 with bolts. The cylinder bracket 79 swingably supports a lift cylinder 34, and corresponds to a lift cylinder supporting member 6 or 106 (see FIG. 1 or FIG. 15) of the present invention described later. An equalizer bar 20 is swingably attached between the front face plate 75 and the rear face plate 76, and the member 77b controls the swinging amount of the equalizer bar 20. The main frame rear portion 73 is constructed of a plate into a box shape.
However, the aforesaid conventional main frame structure and the steering case have the following disadvantages.
(1) An engine, transmission, and various kinds of hydraulic devices and electronic equipment for controlling these devices and the working machine are mounted between the left and right main frames, and these devices and equipment increase in size and component count following the improvement in the various performances and functions of vehicles. Consequently, it is desired to increase the size of the internal width between the left and right main frames. However, if the section of the main frame is constructed into a box shape as in the prior art, or if the plate thickness is increased, the size of the main frame in a width direction is increased, thus causing the disadvantages of increasing the entire width of the vehicle as well as the weight, coupled with an increase in size of the equipment internally provided.
(2) In the invention disclosed in Japanese Utility Model Laid-open No. 2-88877, the cylinder bracket 79 (the lift cylinder supporting element) is provided in the vicinity of the middle of the main frame, specifically on the top of the main frame rear portion 73 which is at the rear of the rear face plate 76 of the cross member 74. The working machine bracket 13 (the working machine frame supporting element) is provided at the lower front portion of the main frame, specifically on the front face plate 75 of the cross member 74 and at the outside of the main frame front portion 72. Accordingly, a horizontal distance L1 (see FIG. 20) between the lift cylinder supporting element and the working machine frame supporting element in side view is larger. As a result, external forces exerted on the blade 31 during operation cause great stress and distortion in the area between the lift cylinder supporting element of the main frame and the working machine frame supporting element, and therefore it is necessary to strengthen this area. However, since the main frame has a structure with a box-shaped section, in order to strengthen the area, the structure is complicated. Thus, it takes longer time to carry out a welding operation, the weight is increased, and the entire width of the vehicle is increased.
(3) The main frame is welded to the steering case provided at the rear portion thereof to be constructed into a single piece, therefore the entire size is considerably large. For this reason, when the mounting faces of the steering case to the power plant, the traveling device, and the like, a large-sized work machine is required, and the operation becomes difficult. In order to avoid the above, if the main frame is welded to the steering case after the completion of the work of each mounting face of the steering case, thermal distortion caused by welding will occur in the steering case. Accordingly, in order to secure working accuracy of the mounting faces, it is necessary to rework the mounting faces. In addition, the steering case, which is a welded structure or a work piece of steel cast, has the disadvantage of having less workability.